In the general art of electric motor control systems, it is well known to derive a feedback signal from the rotating shaft to control a function or operating parameter of the motor. For example, the control system may be intended for speed control; or for electronic or so-called "brushless" commutation; or in the case of a stepping motor, position sensing and control. In order to obtain the desired feedback signal, a signal generating or pick-up sensing means is usually employed. For example, the sensing means may comprise a permanent magnet carried by the shaft and cooperating with one or more semiconductor sensing devices, such as a Hall-Effect generator, fixed with respect to the circular orbit of the rotating magnet. The magnet triggers the Hall-Effect generator to obtain the desired feedback signal, and the signal in turn is fed to suitable electronic circuitry for purposes of controlling a desired function of the motor. In the prior art, of which I am aware, various structural arrangements have been proposed for mounting the semiconductor sensing devices and their cooperating magnets in a variety of applications.
However, none of those structural arrangements are satisfactory for use in high-volume mass-produced portable electric tools intended for consumer as well as professional usage. In this environment, there are a number of unique conditions or criteria to be satisfied, such as size, weight, portability, convenience of usage, and reliability. Portable electric tools have been continually improved for increased power, miniaturization, double-insulation, and cost reduction. As a result, the motor, bearings, and other components are highly-loaded and require proper cooling; and thus the sensing means must not interfere with the flow of cooling air. Moreover, power tools are subject to vibratory forces in usage, especially in the case of a hammer-drill, and to maintain reliable operation the sensing means must be properly mounted to withstand these adverse vibratory forces. The requirements of double insulation (or "D.I.") must also be adherred to, again without interference from the sensing means. Finally, in the very competitive atmosphere of the power tool business, ease of assembly and economy of manufacture are important considerations.